I've not heard of that before but it makes sense to me.
In many retail businesses (whether it's a car dealership, PC World or a furniture store) the saleperson's commission from actually selling the goods is minimal. However what gets him/her far, far more commission is selling the finance deal on the goods.
So a salesperson might be happy to make very little profit on a part exchange deal (or, indeed, on any other retail sale) if he/she's thinking "I'm going to make big money here when it comes to selling the finance". If he/she knows in advance that he/she won't be getting any commission n the finance, he/she's less likely to take a cut in commission from the actual sale.